Method of driving an electroluminescent device

ABSTRACT

A method of driving an organic electroluminescent device comprising a phosphorescent light emitter having an excited state emission decay time τ, which method comprises applying to the organic electroluminescent device a series of electrical pulses of duration t d , such that the ratio t d /τ is less than or equal to 0.1, at a frequency which is less than 1/τ.

The present invention relates to a method of driving an organicelectroluminescent (EL) device.

Emission of a photon from an electronically excited state is referred toas luminescence. Electroluminescence occurs when the excitation isproduced by the application of an electric field. Photoluminescenceoccurs when the excitation is produced by the application of light.Electroluminescence in thin organic films and organic light emittingdiodes (OLEDs) that are based on this phenomenon constitute a rapidlygrowing field of research.

Fluorescence and phosphorescence can be differentiated depending onwhether the transition is between states of equal multiplicity, and thusspin-allowed, or between states of different multiplicity, and thusspin-forbidden. Initial efforts were concentrated on the utilisation offluorescent materials to produce light emission (see C. W. Tang et al.,Appl. Phys. Lett. 51, 913 (1987)). However, although fluorescentmaterials are highly efficient in photoluminescence, only one quarter orso of the total excitations are converted into light in EL devices dueto the spin statistics (see A. R. Brown et al., Chem. Phys. Lett. 210,61 (1993)). The recent success of phosphorescent materials containingtransition metals in overcoming the singlet-triplet bottleneck haspushed up both the practical and theoretical limits of OLED performance(see R. C. Kwong et al., Chem. Mater. 11, 3709 (1999); C. Adachi et al.,Appl. Phys. Lett. 77, 904 (2000); C. Adachi et al., Appl. Phys. Lett.78, 1622 (2001)).

Another group of organic electroluminescent materials which have beeninvestigated are the organolanthanide phosphors (OLPs) (see, forexample, J. Kido et al., Chem. Lett., 657 (1990); J. Kido et al., Chem.Lett., 1267 (1991); S. Capecchi et al., Adv. Mater. 12, 1591 (2000); V.Christou et al., Abstr. Pap. Am. Chem. Soc. 219, U229 (2000)). “OLP” isa UK registered trade mark of Opsys Limited. The OLPs have the samebenefits as phosphorescent materials containing transition metals inconverting both singlet and triplet excitons into photons, and inaddition they have extremely narrow spectral emission (see J. J. Freemanet al., J. Phys. Chem. 67, 2717 (1963)).

In principle, organic electroluminescent devices containing OLP emittersshould have the potential for high efficiency. However, to date thepractical performance of such devices has been limited. When OLPs arecompared with the best phosphorescent materials in EL devices theirefficiency at practical luminance level is significantly belowtheoretical expectations. The maximum brightness achieved from an OLPdevice is much less than from a similar fluorescent EL device.Triplet-triplet (T-T) annihilation has been suggested as the mechanismresponsible for a marked drop in OLP device efficiency at higher currentdensities (see C. Adachi et al., J. Appl. Phys., 87, 8049 (2000)).

Surprisingly, the present inventor has found that the efficiency of anorganic electroluminescent device comprising a phosphorescent emitter ata given current density can be improved relative to the steady statecase by driving the device using electrical pulses which are ofsubstantially shorter duration than the excited state emission decaytime of the phosphorescent emitter.

The present invention accordingly provides a method of driving anorganic electroluminescent device comprising a phosphorescent lightemitter having an excited state emission decay time τ, which methodcomprises applying to the organic electroluminescent device a series ofelectrical pulses of duration t_(d), such that the ratio t_(d)/τ is lessthan or equal to 0.1, at a frequency which is less than 1/τ.

The method of the present invention is applicable to both actively andpassively addressed organic electroluminescent displays. In an activelyaddressed display, each pixel of the display is addressed independently.In a passively addressed display, each row of the display is addressedin turn, so that each row is addressed for only a fraction of the totalframe time. As they are addressed for a smaller proportion of the time,the individual pixels of a passively addressed display must reach ahigher peak brightness in order to give the same overall displaybrightness as a corresponding actively addressed display. A higher peakcurrent density is therefore required. The method of the presentinvention is particularly applicable to passively addressed displaysbecause the improvement in efficiency obtained by using short pulsedriving is particularly significant at higher current densities.

In a conventional passively addressed organic electroluminescentdisplay, the dwell time, t_(d), is typically about 100 μs for a displaywith 100 rows and a refresh rate, f, of 100 Hz. Sufficient light has tobe emitted as a result of this electrical pulse of duration t_(d) forthe average brightness over the whole frame time to be sufficient.

When a fluorescent light emitter is used, the excited state lifetime ofthe emitter, τ, is typically of the order of nanoseconds. This is muchless than the dwell time, t_(d). Light is effectively only emittedduring the voltage or current pulse. In these circumstances, where τ ismuch less than t_(d), for a passively addressed display with N rows, thefollowing equations are found to apply:average brightness=peak brightness/N,average brightness=peak brightness×f×t _(d),  (1)so if N=100, and the desired average brightness is 100 cdm⁻², then thepeak brightness needs to be 10,000 cdm⁻². In this case the averagebrightness is proportional to the dwell time, longer pulses generatingmore light.

Organic electroluminescent displays based on fluorescent light emittingpolymers have been shown to have very high peak brightness when drivenin conventional pulsed mode, and as such they can be used in passivematrix addressed displays. In these materials the light output isproportional to the current even at high current densities, allowing thenecessary high peak brightness to be achieved.

In contrast, when organic electroluminescent devices based onphosphorescent light emitters such as OLPs are run in the steady statemode, the light output is not proportional to the current density; Infact the quantum efficiency, i.e. the ratio of time averaged lightemission to time averaged current density, drops off markedly withincreasing current density. Likewise, when the devices are driven in aconventional pulsed mode with, say, pulses of duration 100 μs, thequantum efficiency decreases markedly with increasing current density.

In view of the marked decrease in quantum efficiency at high currentdensities, phosphorescent compounds have previously been consideredunsuitable as light emitters for displays where high peak brightness isrequired, such as passively addressed organic electroluminescentdisplays. However, in accordance with the present invention, the quantumefficiency may be improved by driving an organic electroluminescentdevice comprising a phosphorescent light emitter using electrical pulseswhich are of short duration compared with the excited state emissiondecay time of the phosphorescent emitter.

Any phosphorescent light emitter may be used in the method of thepresent invention. Phosphorescence generally occurs in complexes wherethere is strong spin-orbit coupling, for example in complexes containinga heavy element, such as a lanthanide metal, or a metal of the second orthird row of the d-block of the Periodic Table (see Inorganic Chemistry,Shriver et al., Oxford University Press, 1990). Examples of suitablemetals for phosphorescent complexes include lanthanide metals such ascerium, samarium, europium, terbium, dysprosium or thulium, and d-blockmetals such as iridium, platinum, rhodium, osmium, ruthenium or rhenium.

Phosphorescent lanthanide metal complexes such as the OLPs generallyrequire one or more sensitizing groups that have the triplet excitedenergy level higher than the first excited state of the metal ion.Emission is from an f-f transition of the metal and so the emissioncolour is determined by the choice of the metal. The emission lifetimesof OLPs are relatively long, making the method of the present inventionparticularly appropriate for this class of compounds. Suitablecoordinating ligands for the lanthanide metals include oxygen ornitrogen donor systems such as carboxylic acids, 1,3-diketonates,hydroxycarboxylic acids, and Schiff bases including acyl phenols andiminoacyl groups.

Some examples of lanthanide metal complexes which may be used in thepresent invention are described in WO 98/55561, WO 00/18851, UK PatentApplication No. 0022081.4 and UK Patent Application No. 0104700.0.

Suitable phosphorescent compounds containing heavy d-block metalsinclude, for example, organometallic complexes with carbon or nitrogendonors such as porphyrin, 2-phenylpyridine, 2-thienylpyridine,benzo(h)quinoline, 2-phenylbenzoxazole, 2-phenylbenzothiazole or2-pyridylthianaphthene. There can also be optional substituents on the(hetero)aromatic rings.

Some examples of phosphorescent compounds containing heavy d-blockmetals which may be used in the present invention are described in U.S.Pat. No. 6,048,630, WO 00/57676, WO 00/70655, WO 99/20081, Pure Appl.Chem. 71 (11), 2095-2106 (1999), and Synthetic Metals, 94, 245-248(1998).

It should be noted that phosphorescence is not necessarily due to smallmolecules. For example, the phosphorescent light emitter used in thepresent invention may be a dendrimer or may be polymeric.

The phosphorescent light emitter used in the present invention ispreferably an organolanthanide phosphor compound, particularly anorganolanthanide phosphor compound of formula (1):M³⁺(L^(n−))_(x)A_(y)  (I)

in which M³⁺ is a trivalent lanthanide metal ion,

L^(n−) is an anionic ligand such that n.x is 3,

A is an electrically neutral co-ligand which may be monodentate orbidentate, and

y is 0, 1 or 2.

In one preferred embodiment, L^(n−) is a 1,3-dicarboxylate ligand offormula (II):

in which R¹ and R², which may be the same or different, are chosen fromalkyl (preferably having from 1 to 6 carbon atoms) which isunsubstituted or is substituted by halogen, aryl (preferably phenyl)which is unsubstituted or is substituted by halogen, thienyl, furanyland pyridyl,

n is 1, x is 3,

M is europium, terbium, samarium or dysprosium, and

A, if present, is a co-ligand such as 1,10-phenanthroline,bathophenanthroline, 2,2′-bipyridyl, a phosphine oxide derivative suchas triphenyl phosphine oxide, water, an N-oxide, a terpyridine, or atetraalkylethylenediamine.

Specific examples of L^(n−) include anions of 2-thenoyltrifluoroacetone(TTA), benzoyltrifluoroacetone (BTFP), dibenzoylmethane (DBM),dithenoylmethane (DTP), and 2-furoyltrifluoroacetone (FTFA).

Specific resulting compounds of formula (1) include europiumtris(2-thenoyltrifluoroacetone) 1,10-phenanthroline (Eu(TTA)₃phen),europium tris(benzoyltrifluoroacetone) 1,10-phenanthroline(Eu(BTFP)₃phen), and europium tris(2-thenoyltrifluoroacetone)bathophenanthroline.

In another preferred embodiment, L^(n−) is a pyrazolone ligand offormula (III):

in which R³, R⁴ and R⁵, which may be the same or different, representhydrogen, an optionally substituted aromatic group or an optionallysubstituted aliphatic or cycloaliphatic group, such that at least one orR³, R⁴ and R⁵ represents a said aromatic group which is conjugated withthe pyrazolone ring system,

n is 1, x is 3, and

A, if present, is a co-ligand such as 2,2′-bipyridyl, or a phosphineoxide derivative (e.g. triphenyl phosphine oxide), or water.

Preferably R³ is a branched alkyl group, R⁴ is methyl, and R⁵ is phenyl.

Specific examples include anions of1-phenyl-3-methyl-4-(2-methylbutan-1-oyl)pyrazolin-5-one,1-phenyl-3-methyl-4-(2,2-dimethylpropan-1-oyl)pyrazolin-5-one and othercompounds described in UK Patent Application No. 0022081.4.

Specific resulting compounds of formula (I) include terbiumtris(1-phenyl-3-methyl-4-(2-methylbutan-1-oyl)pyrazolin-5-one (Tb2B):

and other compounds described in UK Patent Application No. 0022081.4.

Other examples of suitable bidendate anionic ligands, L^(n−), are knownin the literature and include anions of carboxylic acids such as2-(4′-methoxybenzoyl)benzoate. Alternatively multi-dentate ligands suchas the trispyrazolylborate derivatives described in WO 98/55561 can beused.

Other examples of phosphorescent light emitters which may be used in thepresent invention include:

transition metal phosphorescent compounds such as2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) (PtOEP):

compounds described in UK Patent Application No. 0104175.5 such as5,10,15,20-tetra[3′,5′-di(3″,5″-di-tert-butylstyryl)phenyl]porphyrinatoplatinum (II); and

cyclometallated platinum compounds described in WO 00/57676 such ascis-bis[2-(2′-thienyl)pyridinato-N,C³]Pt(II).

In a particularly preferred embodiment of the present invention, thephosphorescent light emitter is Eu(TTA)₃phen. This complex has anexcited state emission decay time τ of about 0.5 ms.

The structure of the organic electroluminescent device used in themethod of the present invention is not particularly limited, as long asit comprises a phosphorescent material as the light emitter. In itssimplest form, the organic electroluminescent device can be formed froman organic layer comprising the phosphorescent material sandwichedbetween two electrodes, at least one of which is transparent to theemitted light. Such a device can have a conventional arrangementcomprising a transparent substrate layer, a transparent electrode layer,a light emitting layer and a back electrode. For this purpose thestandard materials can be used. The transparent substrate layer istypically made of glass although other transparent materials such aspolyethylene terephthalate (PET), acrylic resins and polyamides such asnylon can also be used.

The transparent electrode which typically forms the anode is preferablymade from indium tin oxide (ITO) although other similar materialsincluding indium oxide/tin oxide, tin oxide/antimony and zincoxide/aluminum can also be used. Conducting polymers such as PANI(polyaniline) can also be used.

The back electrode is normally made of a low work function metal oralloy such as Al, Ca, Mg, Li, or MgAg. As is well known, other layersmay also be present, including a hole transporting material and/or anelectron transporting material. In an alternative configuration, thesubstrate may be an opaque material such as silicon and the light may beemitted through the opposing electrode.

In general, if the spontaneous emission of radiation of the appropriateenergy is the only pathway for an excited system with a large number ofindividual emitters to return to its initial state, then the rate ofdecay of emission from the system follows a first-order rate law and maybe expressed as:I=I ₀exp(−k ₀ t),where I₀ is the intensity of radiative emission at time zero, I is theintensity of radiative emission at time t, and k₀ is a constant (see,for example, Excited States and Photochemistry of Organic Molecules, M.Klessinger and J. Michl, VCH, 1995, pages 245-246). In thesecircumstances, the mean natural lifetime of the excited state, τ₀, isgiven by:τ₀=1/k ₀,although, in practice, each process competitive with spontaneousemission reduces the observed lifetime τ relative to the naturallifetime τ₀.

The excited state emission decay time τ of the phosphorescent lightemitter used in the present invention is defined by the equation:I=I ₀exp(−t/τ),where I₀ is the observed intensity of phosphorescent emission from theexcited emitter at time zero and I is the intensity of phosphorescentemission at time t. The excited state emission decay time τ may bemeasured by time-resolved spectrofluorimetry, for example using aHitachi F4500 Scientific Fluorescence Spectrophotometer.

The excited state emission decay time τ of the phosphorescent lightemitter used in the present invention is typically from 0.05 to 1 ms,preferably from 0.25 to 0.75 ms, more preferably about 0.5 ms.

The electrical pulses used in the method of the present invention may bevoltage or current pulses. Current pulses are typically used. The pulsestypically have a substantially rectangular form when the applied voltageor current is plotted as a function of time, although other pulse shapesmay be used. The duration t_(d) is equal to the full width at halfmaximum of the pulse, i.e. the time for the pulse to rise from 50% ofits maximum value to 100% and to fall back to 50%.

Suitable drive pulses are at least an order of magnitude shorter thanthe excited state emission decay time, and are preferably at least twoorders of magnitude shorter. The ratio t_(d)/τ is less than or equal to0.1, preferably less than or equal to 0.05, and more preferably lessthan or equal to 0.01. The pulse duration t_(d) is typically less thanor equal to 50 μs, preferably less than or equal to 10 μs, morepreferably from 1 to 5 μs.

For pulses lasting less than about 5 μs, the brightness of theelectroluminescent emission is found to be proportional to the currentdensity. The current density of the electrical pulses applied accordingto the present invention is not particularly limited, but the pulses aretypically applied at a current density of up to 1 A/cm², preferably 9.1to 500 mA/cm², more preferably 0.1 to 100 mA/cm².

For pulses lasting less than about 5 μs, the brightness is also found tobe proportional to the pulse duration. However, when longer pulses areused, the quantum efficiency is not maintained (see FIGS. 5, 8 and 9 ofthe accompanying drawings). This is quite unlike the case withfluorescent emitters, where the quantum efficiency is independent of thepulse duration.

Although the drive pulses used according to the present invention areshort, the relatively long excited state lifetime means that lightcontinues to be emitted after the drive pulse has finished. Thefrequency at which the pulses are applied is less than 1/τ, preferablyless than 0.5/τ, more preferably less than 0.1/τ. The frequency istypically from 10 Hz to 1 kHz, preferably from 20 to 200 Hz, morepreferably from 50 to 100 Hz.

In a test device run at 100 Hz with 5 μs duration pulses, the averagebrightness of the device was 10 cd/m². As shown in FIG. 7 of theaccompanying drawings, increasing the refresh rate, for example to 200,500 or 1000 Hz, does not change the size or shape of theelectroluminescent transient. However, the number of pulses per secondis higher each time the refresh rate is increased, and hence the averagebrightness of the device is proportionately greater. Using short pulsesat a moderate frequency thus provides a viable route to sufficientlybright devices.

It has previously been proposed that the decrease in efficiency withincreasing current density in OLP EL devices is due to triplet-tripletannihilation. Triplet-triplet annihilation is a bimolecular process andhence is more pronounced at higher concentrations of triplets, forexample at higher current density. In the short pulses used in thepresent invention the current density during the pulse is typically veryhigh and a high density of triplets should therefore be formed.Unexpectedly, however, the sharp drop in efficiency that would indicatetriplet-triplet annihilation is not seen (see FIGS. 4 and 6 of theaccompanying drawings).

Previous passive matrix drive schemes have been developed forfluorescent systems with the refresh rate determined by the number ofrows. However, when the emitter has a long excited state relative to thedwell time, then equation (1) is not appropriate. Instead, when t_(d) ismuch less than τ, assuming instantaneous charging of the excited state,theory predicts that the average brightness of a passive matrix displayis:average brightness=peak brightness×f×τ,  (2)which depends on the excited state lifetime rather than the dwell time.This equation should be borne in mind when designing a passive matrixaddressing scheme for an organic electroluminescent device comprising aphosphorescent light emitter. However, the choice of pulse duration isalso of relevance as the devices are more efficient when driven withshorter pulses.

The present invention will be further described in the Examples whichfollow, with reference to the accompanying drawings in which:

FIG. 1 illustrates the steady state current-voltage-luminance (J-V-L)characteristics of the organic electroluminescent device prepared inReference Example 1;

FIG. 2 illustrates the transient EL emission from the device prepared inReference Example 1 with a fixed current density of 500 mA/cm² and apulse duration varying from 10 μs to 1 ms;

FIG. 3 illustrates the transient EL emission from the device prepared inReference Example 1 with a fixed pulse duration of 10 μs and a currentdensity varying from 32.4 to 324 mA/cm²;

FIG. 4 illustrates the dependence of quantum efficiency on averagecurrent density for the device prepared in Reference Example 1 withsteady state driving (triangles) and pulsed driving (circles);

FIG. 5 illustrates the dependence of quantum efficiency on pulseduration for the device prepared in Example 2;

FIG. 6 illustrates the dependence of quantum efficiency on currentdensity for the device prepared in Example 2 with both steady state andpulsed driving;

FIG. 7 illustrates the transient EL emission from the device prepared inExample 2 with a fixed pulse duration of 5 μs and a refresh rate varyingfrom 100 to 1000 Hz;

FIG. 8 illustrates the dependence of quantum efficiency on pulseduration for the device prepared in Example 3; and

FIG. 9 illustrates the dependence of quantum efficiency on pulseduration for the devices prepared in Example 4 (circles) and ComparativeExample 1 (triangles).

REFERENCE EXAMPLE 1

Preparation of Organic Electroluminescent Device

Indium tin oxide (ITO) coated glass supplied by the Applied FilmsCorporation was patterned by standard photolithography to produce a setof ITO stripes. The patterned substrates were sonicated in detergent,thoroughly rinsed with de-ionised water, blown with dry nitrogen andcleaned with oxygen plasma immediately before loading into a vacuumchamber. The base pressure of the vacuum system used for devicefabrication (SPECTROS, KJ Lesker Limited, UK) was lower than 10⁻⁷ Torr.The device structure consisted of 50 nm of4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (α-NPD) as a holetransporting layer, 40 μm of2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (butyl-PBD)doped with 7.6 mol % of europium tris(2-thenoyltrifluoroacetone)1,10-phenanthroline (Eu(TTA)₃phen) as an emitting layer, 60 nm of2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) doped with 50 mol %of Li as a hole blocking/electron transporting layer, and 100 nm of Alas a cathode.

REFERENCE EXAMPLE 2

Steady State Characteristics

The steady state J-V-L characteristics of the device prepared inReference Example 1 are shown in FIG. 1 of the accompanying drawings.The device has a rather low quantum efficiency, the maximum beingapproximately 0.3 cd/A at 0.13 mA/cm². Although inefficient, the deviceis stable in ambient conditions under DC currents of up to 1 A/cm² andsuch currents can be reached at relatively low driving voltages.

EXAMPLE 1

The device prepared in Reference Example 1 was tested under pulsedcurrent driving, using an AVTECH AV-1011B1-B pulse generator, aTektronix TDS 3054 500 MHz digital storage oscilloscope and a Siphotodiode were used. The time response of the system was better than 1μs. A repetition frequency of 100 Hz (10 ms period) was chosen for theseexperiments as a typical display refresh rate.

The results of the pulsed driving are presented in FIGS. 2 and 3. InFIG. 2 the magnitude of the current pulse was kept constant at 500mA/cm² while the duration of the pulse was varied from 0.01 to 1 ms. InFIG. 3 the current pulse duration was fixed at 0.01 ms and the magnitudeof the current pulse was varied from 32.4 to 324 mA/cm².

Whilst transient EL in the fluorescent materials closely follows theshape of the driving current pulse, the EL rise and the fall times forboth transition metal and lanthanide phosphorescent materials depend onthe phosphorescence lifetime. As seen from FIGS. 2 and 3, a longafterglow that lasts longer than 2 ms is present in the EL transient.This afterglow is a result of the 0.5 ms radiative lifetime of theexcited Eu³⁺ ion in the Eu(TTA)₃phen complex.

The amount of light generated per pulse was calculated by measuring thearea under the transient EL signal. Relative quantum efficiencies wereestimated from the ratio of the area of the EL transient to the area ofthe current pulse. According to the data, the efficiency of the ELemission goes up by an order of magnitude while the duration of theapplied 500 mA/cm² current pulse is reduced from 1 ms to 10 μs. When theduration of the current pulse was fixed at 10 μs and the magnitude ofthe current pulse was varied from 32 to 324 mA/cm², the relative ELefficiency at high current densities dropped by a factor of two.Efficiency curves are presented in FIG. 4, where the results obtainedwith steady state and pulsed driving are represented as triangles andcircles, respectively. It can be seen that the pulsed mode pulsed modeefficiency plotted as a function of average current density (peakcurrent density corrected for the duty ratio) is higher than the steadystate EL efficiency by one order of magnitude.

EXAMPLE 2

An organic electroluminescent device was prepared containingEu(TTA)₃phen as the phosphorescent light emitter. The device consistedof 4,4′-bis(carbazole-9-yl)biphenyl (CBP) doped with Eu(TTA)₃phen as anemitting layer, BCP as a hole blocking/electron transporting layer, andLiF/Al as a cathode.

20V pulses were applied to the device. FIG. 5 shows the dependence ofquantum efficiency on pulse duration at a fixed current density of 400mA/cm², while FIG. 6 shows the dependence of quantum efficiency oncurrent density for both steady state and pulsed driving.

FIG. 7 shows the effect on the transient EL emission of varying thefrequency from 100 to 1000 Hz while fixing the pulse duration at 5 μs.

EXAMPLE 3

An organic electroluminescent device was prepared containing terbiumtris(1-phenyl-3-methyl-4-(2-methylbutan-1-oyl)pyrazolin-5-one (Tb2B) asthe phosphorescent light emitter. The device consisted of 20 nm thickCBP doped with Tb2B at about 10 weight % as an emitting layer, 60 nm ofBCP as a hole blocking/electron transporting layer, and 1.2 nm LiF/100nm Al as a cathode.

20V pulses were applied at a frequency of 100 Hz. The dependence ofquantum efficiency on pulse duration is shown in FIG. 8.

EXAMPLE 4

An organic electroluminescent device was prepared containing2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) (PtOEP) asthe phosphorescent light emitter. The device consisted of 20 nm thickCBP doped with PtOEP at about 10 weight % as an emitting layer, 60 nm ofBCP as a hole blocking/electron transporting layer, and 1.2 nm LiF/100nm Al as a cathode.

20V pulses were applied at a frequency of 100 Hz. The dependence ofquantum efficiency on pulse duration is shown by the results plotted ascircles in FIG. 9.

COMPARATIVE EXAMPLE 1

An organic electroluminescent device was prepared containingtris(8-quinolinolato)aluminium (III) (Alq₃) as a fluorescent lightemitter. The device consisted of 50 nm of α-NPD as a hole transportinglayer, 50 nm Alq₃ as an emitting layer, and 1 nm LiF/100 nm Al as acathode.

14V pulses were applied at a frequency of 100 Hz. The dependence ofquantum efficiency on pulse duration is shown by the results plotted astriangles in FIG. 9.

1. A method of driving an organic electroluminescent device comprising aphosphorescent light emitter having an excited state emission decay timeτ, which method comprises applying to the organic electroluminescentdevice a series of electrical pulses of duration t_(d), such that theratio t_(d)/τ is less than or equal to 0.1, at a frequency which is lessthan 1/τ.
 2. A method according to claim 1, wherein t_(d)/τ is less thanor equal to 0.05.
 3. A method according to claim 1, wherein τ is from0.05 to 1 ms.
 4. A method according to claim 1, wherein t_(d) is lessthan or equal to 5 μs.
 5. A method according to claim 1, wherein theelectrical pulses are applied at a current density of from 0.1 to 100mA/cm².
 6. A method according to claim 1, wherein the electrical pulsesare applied at a frequency of from 10 Hz to 1 kHz.
 7. A method accordingto claim 1, wherein the phosphorescent light emitter is anorganolanthanide phosphor compound.
 8. A method according to claim 7,wherein the organolanthanide phosphor compound is a compound of formula(I):M³⁺(L^(n−))_(x)A_(y)  (I) in which M³⁺ is a trivalent lanthanide metalion, L^(n−) is an anionic ligand such that n.x is 3, A is anelectrically neutral co-ligand which may be monodentate or bidentate,and y is 0, 1 or
 2. 9. A method according to claim 8, wherein theorganolanthanide phosphor compound is europiumtris(2-thenoyltrifluoroacetone) 1,10-phenanthroline (Eu(TTA)₃phen). 10.A method according to claim 8, wherein the organolanthanide phosphorcompound is terbiumtris(1-phenyl-3-methyl-4-(2-methylbutan-1-oyl)pyrazolin-5-one (Tb2B).11. A method according to claim 1, wherein the phosphorescent lightemitter is 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II)(PtOEP).
 12. A method according to claim 1, wherein the organicelectroluminescent device is passively addressed.